1. Field of the Invention
The present invention relates generally to mobile telephone systems, and more particularly to a probability based lock detection method and system for mobile telephone systems.
2. Related Art
Mobile telephone systems allow customers to place telephone calls from wireless devices referred to as mobile telephones or subscriber units. The mobile telephone transmits a signal to a base station. The base station is interconnected to a mobile telephone switch. The mobile telephone switch interconnects the base stations to each other and to public switched telephone networks (PSTNs).
One method that is used for mobile telephone transmission to a base station is via a ground-based antenna that operates in UHF band. This is the same band used for broadcast television transmission. Use of this method limits the subscriber to communication within a cell which is the serving area that the antenna can transmit using UHF band. Subscribers can move from cell to cell because handoffs are possible from one cell to another. However, if no ground-based antenna is within a distance that can be reached using UHF band, such as in a rural area, a subscriber cannot use the mobile telephone.
Developments in mobile telephone system technology have led to mobile telephone systems that can transmit using a low earth orbit (LEO) satellite systems. The mobile telephone systems that use LEO satellite systems can transmit to rural areas because the subscriber does not need to be within a close range of the ground-based antenna. As a result, mobile telephone systems using LEO systems are not limited to major cities as are mobile telephone systems that use ground-based antennas which operate in the UHF band.
The transponder is the component in a satellite that receives and transmits signals from subscribers using mobile telephones. A satellite transponder must be able to carry calls from a large number of subscribers simultaneously in order to be cost effective. Various satellite access schemes such as time division multiplex access (TDMA) and code division multiplex access (CDMA) allow simultaneous access to transponders by a large number of subscribers.
Digital CDMA is preferable to other satellite access schemes as more customers can be carried at a lower cost and higher quality. Low powered signals allow transmission of CDMA signals via small, inexpensive antennas requiring less expensive earth station and network equipment than other satellite access systems. In order to carry the highest number of customers on a CDMA channel, the power must be used efficiently.
In a CDMA system, data from each customer is carried on an individually encoded, modulated, and PN spread channel which is referred to as a traffic channel. Different PN spreading codes are used for different users so that multiple users can share the same wideband channel which is also referred to as a CDMA channel.
Because multiple customers or users share the same wideband channel, if one customer or user'signal is transmitted at a higher power than the other customers or users on the channel, interference may occur which may result in unacceptable performance unless the number of users on the wideband channel is reduced. In addition, lower power transmission helps overcome fading because signals can be spread through more of the wideband channel and more capacity is available in the wideband channel for diverse paths. Lower power transmission also conserves power at the base station. However, if the power of a customer'signal becomes too low, the quality of service for that customer becomes unacceptable.
In order to ensure a high quality of service, the lock status of the received signal is monitored. A signal is in lock if it is strong enough to be received by the base station. A signal is out of lock if it is not strong enough to be received by the base station. Lock detection may be performed by comparing the energy of a Walsh symbol or the average energy of several consecutive Walsh symbols (also referred to as the longterm average energy) with a threshold. If the energy exceeds the threshold, the signal is in lock. If the energy falls below the threshold the signal is out of lock. Walsh symbols, which are described in more detail below, are components of the signal transmitted by the mobile unit. Lock detection is described in more detail in U.S. Pat. No. 5,654,979, entitled "Cell Site Demodulation Architecture For a Spread Spectrum Multiple Access Communication Systems," issued Aug. 5, 1997 to the assignee of the present invention and incorporated herein by reference.